New regulations have been established to protect the stratospheric ozone layer from possible damage by chlorofluorocarbons (CFCs). Highly purified HFC-32 is a hydrofluorocarbon (HFC) that is valuable as an etchant gas in plasma etching of materials used in the fabrication of semiconductor devices.
HFC-32 may be prepared by allowing methylene chloride (CCl2H2) to react with hydrogen fluoride (HF) in the presence of an oxidized metal catalyst of metals such as chromium, antimony, and tantalum. HFC-32 may also be co-produced with HFCs such as pentafluoroethane (CF3CF2H, HFC-125) by such metal mediated processes in which an HFC-125 precursor compound such as tetrachloroethylene (CCl2═CCl2) is utilized. The HFC-32 reaction product obtained from such processes may contain one or more of HFC-125, unreacted methylene chloride and HF, byproduct hydrogen chloride (HCl), and small amounts of organic byproducts such as 1,1,1-trifluoroethane (CF3CH3, HFC-143a), dichlorodifluoromethane (CCl2F2, CFC-12), chloropentafluoroethane (CClF2CF3, CFC-115), methyl chloride (CH3Cl, HCC-40), methyl fluoride (CH3F, HFC-41), trifluoromethane (CF3H, HFC-23), chlorodifluoromethane (CHClF2, HCFC-22), and 1,1-difluoroethane (CF2HCH3, HFC-152a). The presence of even trace amounts of such impurities in HFC-32 can be undesirable in the utilization of HFC-32 as an etchant gas in plasma processes employed in the semiconductor industry.
Casey et al., in PCT publication WO9703936 disclose processes for separation of HFC-32 and HFC-125 by azeotropic distillation of a low boiling HFC-32/HFC-125 azeotrope, separation of a mixture of HFC-32 and CFC-115 by azeotropic distillation of a low boiling HFC-32/CFC-115 azeotrope, separation of a mixture of HFC-32 and HFC-125 by extractive distillation employing methylene chloride as extractive agent, and separation of a mixture of HFC-32 and HFC-143a by employing CFC-115 as extractive agent.
Takahashi Reiji et al., in Japanese patent application JP 07291878, describe a process for the removal of HFC-143a, HFC-125, and methyl chloride from HFC-32 by extractive distillation. This process is characterized by employing at least one of 1,1-dichloro-1-fluoroethane (CCl2FCH3, HCFC-141b), dichloropentafluoropropane, trichlorotrifluoroethane, and 2,2-dichloro-1,1,1-trifluoroethane (CHCl2CF3, HCFC-123) as extractive agent. Using such CFC extractive agents is relatively expensive, and regulations concerning protection of the stratospheric ozone layer will cause CFCs to be phased out as commercial products thereby making CFCs unavailable or uneconomical for such a process.
The present invention solves problems associated with conventional purification methods and provides lower cost and more economical processes for separating HFC-32 from mixtures comprising HFC-32 and at least one halocarbon selected from HFC-143a, CFC-12, HFC-125, and CFC-115.